To make the levain: In a medium bowl, combine the water and sourdough culture. Mix with your hands and fingers until the culture is broken up and well distributed in the water., Add the flour and salt. Mix briefly, then knead until smooth. Cover and set at room temperature (about 68°F) for 12 to 16 hours., To make the dough: In a large mixing bowl, combine the dough water and stiff levain., Mix with your hands until the levain is broken up in the water, then add the flours, salt, and yeast. Stir with the handle end of a wooden spoon until the dough forms a shaggy mass. If you find it easier, after stirring some, scrape the dough out of the bowl with a plastic scraper onto your work surface and knead briefly with your hands just until the dough comes together. Resist the urge to add flour., Scrape the dough off the work surface and return to the bowl for bulk fermentation., Scrape down the sides of the mixing bowl and allow the dough to rise, covered, for 3 hours at room temperature. While it rises, fold after 15, 30, 45, 60, and 120 minutes. To fold dough, bring the outside edge closest to you into the center. Rotate the bowl 90°; bring the outside edge closest to you into the center. Repeat twice more: this counts as one fold., After the final fold, leave untouched for the last hour. As you perform each series of folds, you'll begin to notice that the dough becomes smoother, stronger, and more cohesive., Pre-shape the dough into a round loaf. Cover and let rest for 15 minutes., Uncover the dough and shape it into a smooth, round boule. Place the boule seam-side up in a floured banneton or floured, towel-lined bowl, approximately 10" wide and 4" deep. A 9" lined brotform should work just fine., Cover and let rest for 50 to 60 minutes at room temperature., Toward the end of this final rest, preheat the oven to 450°F with a lidded cast iron pot and lid in the oven. Our bread and potato pot is a good choice here., Transfer the loaf to a parchment sling, gently inverting it so that the side that was against the dusted towel — the smooth side — becomes the top., Score the bread several times with a lame and gently transfer to the pot, sling and all. The sling will remain underneath the loaf as it bakes., Put the lid on the pot and bake the bread for 40 to 45 minutes, removing the lid after 30 minutes. The loaf should be a deep golden brown by the end of its bake., Remove the bread from the oven, and cool it on a rack. Don't slice until it's fully cooled., Store bread in a paper bag at room temperature for several days. Freeze for longer storage.